1. Field of the Invention
This invention relates generally to materials formed of fluorinated resin powders and particularly relates to sealing elements formed of sintered fluorine based polymers such as polytetrafluoroethylene.
2. Description of Prior Developments
Polytetrafluoroethylene (PTFE) is commonly used in the manufacture of sealing elements such as the annular disk-shaped seal lips of radial lip oil seals. PTFE is chosen for oil seal applications because it has relatively good wear resistance and elasticity at operating temperatures up to about 400.degree. F. PTFE also has good resistance against chemical attack by oils and other liquids frequently present in oil seal applications.
Disk-type sealing elements have been made by compacting PTFE powder into a tubular preform which is sintered to form a billet. The billet is then sliced along radial planes so as to form a plurality of annular disks with each disk constituting one sealing element. In order to improve the performance of PTFE sealing elements, various fillers in finely divided form are sometimes added to the PTFE powders prior to their compaction and sintering. Commonly, such fillers are finely dispersed powdered fiberglass, molybdenum disulphide, and/or graphite. Fillers can improve the wear resistance and compressive strength of the sealing elements while adding lubricity to its sealing surfaces.
When disk-shaped sealing elements containing fillers are formed by slicing through a tubular billet, each disk-shaped slice will have finely dispersed fillers uniformly distributed throughout its entire radial dimension, from its inner circumferential edge to its outer circumferential edge. The type of finely dispersed fillers typically needed at or near the contact sealing surface of the disk element are not the same type required or desirable for use in the area of the disk element used for mounting the element on a support member. Prior manufacturing techniques could not adequately provide for the selective addition of one or more fillers to different portions of a polymeric sealing element.
Since the fillers are typically less expensive than pure fluorine based polymers, cost savings can be realized by filling the mounting portion with inexpensive fillers. It is thus desirable from a cost consideration to be able to form an annular sealing element with one type of filler provided along its sealing surface which undergoes relative dynamic contact, and another type of economical filler provided in its static mounting portion. That is, the greater the amount of inexpensive filler provided in the mounting portion, the less costly will be the sealing element.
Although it is possible to form two separate sintered billets with different fillers in each billet, and to subsequently join the billets in a nested concentric configuration, the joining of the separate pre-sintered billets requires a second sintering operation. In the case of sealing elements, a second sintering generally causes a loss of wear resistance. This is due to the molecular degradation which takes place during the second sintering wherein the long polymer chains are broken into shorter chains.
Another drawback associated with double sintering is the necessity to apply compaction pressure in a mold press to the once sintered billets during the second sintering. This requires the mold press to be maintained in a heated state during the second sintering. This type of process consumes a great deal of mold time and is thus not well suited to high volume production and the efficient use of mold time.
A process somewhat related to the present invention is disclosed in U.S. Pat. No. 4,243,235 to J. Repella wherein a composite sealing element is formed with an elastomer-to-PTFE adhesive bond. In this process, an axially elongated composite tubular billet is created by first placing a sintered PTFE tube in an oversized cylindrical mold cavity. One surface of the PTFE tube is etched and coated with an adhesive. An annular space between the adhesively-coated surface of the PTFE tube and the facing mold cavity surface is then filled with an elastomer.
The elastomer can be a resilient plastic such as rubber or polyurethane which can be injected into the mold cavity or placed in the cavity as a precharge. The elastomer can be pre-cured or uncured, i.e. vulcanized during the molding operation. Heat and pressure are applied to the mold to form a tubular billet having a bond formed at the annular interface between the PTFE tube and the elastomeric annulus. Multiple sealing disk elements are formed by slicing through the plastic and elastomeric tubular billet along radial planes.
During service, a composite plastic-elastomeric sealing disk of the type disclosed in U.S. Pat. No. 4,243,235 may fail due to differential thermal expansion of the different materials (plastic and elastomer) along their bond interface. That is, differences in the expansion rate and contraction rate of the two different materials can, in time, weaken or impair the bond at the annular interface between the two materials. Flexure of the plastic section relative to the elastomeric section can also overstress the joint between the two sections.
By heating the PTFE a second time during bonding of the PTFE to the elastomer, the crystallinity of the PTFE may be adversely affected. This can reduce the wear resistance of the PTFE and thus adversely affect the performance of the sealing element. Moreover, elastomeric materials do not possess the ability to withstand temperatures as high or as low as PTFE, nor can elastomers resist chemical attack as well as PTFE and its related polymers.
Accordingly, a need exists for a low cost composite sealing element which minimizes differential thermal expansion between materials, which requires no additional adhesives or pre-bonding surface preparation such as chemical etching and which minimizes molecular degradation due to multiple sinterings.